Marina Litvinenko, widow of Alexander Litvinenko (a British citizen who was assassinated in London by two former KGB agents who poisoned him with radioactive polonium) has accused the British government, Secretary of State William Hague, and PM David Cameron of sabotaging the coroner's inquest into her husband's death. Hague and Cameron intervened in the coroner's hearing to seal key evidence that implicated the Russian government in Litvinenko's killing.

Sir Robert Owen, who is leading the inquest and who has seen the material, characterised it as "documents that examined whether UK officials could have done more to prevent his murder." 's widow says that this is part of "a secret political deal with the Kremlin." This comes against a charm offensive by the UK government to increase Russian investment in Britain.

The former Labour government severed all contacts with Russia's FSB spy agency in 2007 after concluding it had played a leading role in Litvinenko's assassination. Putin is the agency's former chief.

Mrs Litvinenko added: "This is a very sad day, a tragedy for British justice which has until now been respected around the world, and a frightening precedent for all of those who have been trying so hard to expose the crimes committed by a conspiracy of organised criminals who operate inside the Kremlin."

In his ruling (pdf), Owen said the inquest scheduled to take place later this year might now result in an "incomplete, misleading and unfair" verdict.

The coroner said he would consider inviting Theresa May, the home secretary, to hold a public inquiry instead. The inquiry could hear the sensitive evidence buried by Hague in secret sessions.